Nashville, home of the Grand Old Opry and the Brand-New Parthenon, was founded on a low hill overlooking the Cumberland River in 1779. Well situated on a deepwater port with an easy float to the Mississippi River, at the northern terminus of the Natchez Trace to walk home again, the town grew quickly. Nashville was chartered as a city shortly after Tennessee was granted statehood in 1796, and selected as state capitol in 1843, thanks in large part to her favorite son, Andrew Jackson.
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| Pleurocera laqueata [1] |
Meanwhile, back in New Harmony, his buddy Thomas Say kept the printing presses cranking. And in 1829 Say described a pleurocerid snail named Melania laqueata, as follows [1]:
“Shell oblong: spire longer than aperture, elevated, conic, acute: volutions moderately convex, with about seventeen regular, elevated, equal, equidistant costae on the superior half of each volution, extending from suture to suture, and but little lower, and becoming obsolete on the body whirl; suture moderately impressed; sinus obsolete. This species was found by Dr. Troost in Cumberland River. Aside from a difference in form, it may be distinguished from cancellata, nob., and catenaria, nob., by being altogether destitute of elevated revolving lines. The young shell is carinated.”
Today, of the (roughly 1,000) names for species of pleurocerid snails described from the waters of North America, Thomas Say’s “Melania laqueata” is twelfth oldest [2]. And populations matching the snails that Gerard Troost sent to Thomas Say from the “Cumberland River,” reidentified as “Goniobasis” laqueata between 1862 and 1980, re-reidentified as “Elimia” laqueata 1980 – 2011, re-re-reidentified as Pleurocera laqueata in the modern day [3], have turned out to be common and widespread in rivers and streams throughout the greater Cumberland and Green River drainages, the upper Kentucky River, and Tennessee River drainages west of Chattanooga.
So, “The Cumberland River” is a big place. Who could honor the Volunteer Spirit of Tennessee better than a malacologist stepping forward to narrow down (or “restrict’) Thomas Say’s type locality for Melania laqueata to some more precise spot? And one’s natural first thought – correct me if I am wrong – would be to assume that Gerard Troost collected that first specimen of M. laqueata from the Cumberland River as it runs by his adopted home of Nashville. But alas.
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| Modern Nashville |
Efforts by the U.S. Army Corps of Engineers to blast the Cumberland River clear of obstacles to navigation began as early as the 1830s. The first lock and dam on the Cumberland River was constructed at Nashville in 1887, and by the 1920s a system of 15 locks and dams regulated the Cumberland River to a minimum depth of 6 feet through the entire state of Tennessee. Attention then turned to the generation of hydroelectric power, the COE constructing a series of gigantic dams in the 1940s through the 1970s, including Old Hickory Dam just 20 river miles upstream from Nashville in 1956.
A visit to the Nashville waterfront today betrays no hint of gastropod habitat, nor indeed, home for macrobenthic life of any sort or description. Downstream the Cumberland River is armored with rip rap boulders. Upstream the flow is increasingly controlled by the generation schedule at the Old Hickory Dam, daily cycles at the Edenwold Gage often reaching amplitudes of 6 feet. Slackwater extends 100 miles above the dam, essentially to the base of Lake Cordell Hull, which extends another 70 miles, essentially to Kentucky. If a viable population of pleurocerid snails of any description survives in the Cumberland River of Tennessee today, I am not aware of it.
It seems to me that we are left with no alternative but to select a tributary of the Cumberland River as the type locality for Thomas Say’s Melania laqueata. And the tributary closest to Gerard Troost’s base of operations currently inhabited by a viable population of pleurocerid snails matching Thomas Say’s 1829 description would be Browns Creek, a small stream running north through the state fairgrounds to empty into the Cumberland entirely within the modern city limits of Nashville.
I visited Browns Creek at the state fairgrounds on a sunny Saturday morning this April just past, as crowds were beginning to gather for the INEX Spring Nationals at the Fairgrounds Speedway [4]. If you click the image below for an enlargement, you can see a supertruck practicing on the track below, at far left. Browns Creek runs under that bridge I’ve marked with an arrow.
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| Tennessee State Fairgrounds |
The stream itself doesn’t stink anywhere near as bad as you might expect from its entirely urban catchment. Sure, there was garbage and litter of all sorts everywhere down in the rather narrowly incised ditch through which Browns Creek runs. But the water was clear, and coolish for April, and running over riffles, and you could flip rocks and find mayfly larvae. I’ve waded into much worse.
The pleurocerids were not abundant, but with an hours’ effort I was able to collect N = 29 topotypic Pleurocera laqueata laqueata (Say 1829) from Browns Creek at the state fairgrounds, in Nashville, TN (36.1282, -86.7628). At this point I propose to restrict the type locality of Melania laqueata Say 1829.
My sample demonstrated the range of shell morphological variation typical of pleurocerid populations everywhere. But before we follow that thread any further, we need to clarify some terminology.
In his original description, Thomas Say focused on the “regular, elevated, equal, equidistant costae” on the whorls of the shell. Such scallop-shaped ridges on the whorls have also been called, by other authors at other times, “costations,” “plicae,” or “plications.” Generally, in previous posts on this blog, I have tended to prefer plications (adj. plicate) to describe that particular shell feature, so let’s try to be consistent.
And Thomas Say also went on to stipulate that the shell of his Melania laqueata was “altogether destitute of elevated revolving lines.” Such shell features have also been called, by other authors at other times, “spiral lines” or “spiral cords” or “striae” or “striations.” I have generally preferred striation (adj. striate) in past essays on this blog, so again, let’s stick with that. Thomas Say’s holotype shell figured way up above demonstrates very strong plications but no striation whatsoever.
So a small sample of the shells born by the newly designated topotypic population of P. laqueata is figured below. And it should come as no surprise to see significant intrapopulation variation in shell plication. All are plicate around the apex, but the body whorl of shell on the left is essentially smooth, that of the shell on the right strongly plicate, and the shell in the middle approximately half-plicate, around the top of the body whorl only.
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| Topotypic P. laqueata |
The subject of shell plication in pleurocerid snails has come up at least three times previously in the columns of this blog, maybe more [5], most recently in an essay I published on P. troostiana back in [15Apr20]. My loyal and attentive readership may recall that Calvin Goodrich devoted #3 in his “Studies on the Pleuroceridae” series to plication way back in 1934 [6]. The laboratory rearing experiments of Misako Urabe [7] returned evidence that at least some variation in the strength of shell plication may be an ecophenotypic response to substrate.
And we shouldn’t let this opportunity pass to tip our caps to Thomas Say, the Father of American Malacology, as well. In a quaint nineteenth-century fashion, I think he may be trying to telegraph that he noticed intrapopulation variance in the plication of Melania laqueata, like a Charles Darwin on the American frontier. His figured holotype clearly shows strong plication (“costae”) across the entire body whorl “from suture to suture,” much like topotypic shell C above. But in his description, he specified:
“seventeen regular, elevated, equal, equidistant costae on the superior half of each volution, extending from suture to suture, and but little lower, and becoming obsolete on the body whirl.”
The wording of Say’s written description about plication on “the superior half of each volution … and but little lower” implies to me a morphology more like topotypic shell B. And that final clause about plication “becoming obsolete on the body whirl” suggests more the morphology demonstrated by topotypic shell A.
Darwin’s theory depended on three hypotheses: that populations vary, that such variation yields fitness differences, and that fitness differences drive evolution. The first hypothesis is the easiest to test, but historically, was the most difficult to accept. It is humbling to see a pre-Darwinian systematic biologist such as Thomas Say entertaining an hypothesis that so many 21st-century systematic biologists refuse to consider.
Ah, but. Thomas Say was very, very certain that the shell of his new Melania laqueata was “altogether destitute of elevated revolving lines.” What is the situation with striation? Tune in next time.
Notes:
[1] Say, T. (1829) Descriptions of some new terrestrial and fluviatile shells of North America. New Harmony Disseminator of Useful Knowledge 2(18): 275 – 277.
[2] Melania laqueata is in a five-way tie for twelfth oldest, to be precise, with the four other pleurocerids described by Thomas Say in 1829: semicarinata, obovata, canaliculata, and trilineata.
[3] The history of the genus of pleurocerid snails to which Say’s Melania laqueata has been assigned is long and tortured. For a brief review, see:
- Goodbye Goniobasis, Farewell Elimia [23Mar11]
[4] The Nashville Fairgrounds Speedway is the second oldest continually operating motorsports track in the United States. It hosted Grand National / Winston Cup NASCAR races 1958 – 1984, and NASCAR Busch Series races 1984 – 2000, before being replaced on the schedule by the 1.33 mile Nashville Superspeedway in 2001. Here’s a quote from the sportscaster Dave Moody (interviewing Sterling Marlin): “If they announced that five old ladies would push baby buggies around that track, 4,000 people would show up.”
[5] Previous essays touching on shell plication in the Pleuroceridae:
- Semisulcospira research: A message from The East [6Jan08]
- Semisulcospira research: A second message from The East [1Feb08]
- What is a subspecies [4Feb14]
- What subspecies are Not [5Mar14]
- Huntsville Hunt [15Apr20]
[6] Goodrich, C. (1934) Studies of the gastropod family Pleuroceridae – III. Occasional Papers of the Museum of Zoology, University of Michigan 300: 1 – 11.
[7] Urabe, M. 2000. Phenotypic modulation by the substratum of shell sculpture in Semisulcospira reiniana (Prosobranchia: Pleuroceridae). J. Moll. Stud. 66: 53-59.